Problem: Divide the following complex numbers: $\dfrac{9(\cos(\frac{2}{3}\pi) + i \sin(\frac{2}{3}\pi))}{\cos(\frac{7}{4}\pi) + i \sin(\frac{7}{4}\pi)}$ (The dividend is plotted in blue and the divisor in plotted in green. Your current answer will be plotted orange.)
Solution: Dividing complex numbers in polar forms can be done by dividing the radii and subtracting the angles. The first number ( $9(\cos(\frac{2}{3}\pi) + i \sin(\frac{2}{3}\pi))$ ) has angle $\frac{2}{3}\pi$ and radius 9. The second number ( $\cos(\frac{7}{4}\pi) + i \sin(\frac{7}{4}\pi)$ ) has angle $\frac{7}{4}\pi$ and radius 1. The radius of the result will be $\frac{9}{1}$ , which is 9. The difference of the angles is $\frac{2}{3}\pi - \frac{7}{4}\pi = -\frac{13}{12}\pi$ The angle $-\frac{13}{12}\pi$ is negative. A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $-\frac{13}{12}\pi + 2 \pi = \frac{11}{12}\pi$ The radius of the result is $9$ and the angle of the result is $\frac{11}{12}\pi$.